Wow, this is a tough question. Reviewing surface finish with an optical microscope is pretty basic and limited to such items as surface scale and major surface imperfections such as pitting, large grooves, gouges, rolled in seams and other fairly large mechanical/chemical surface defects. In other words, it is limited to those defects you can see and measure with a microscope.

Surface finish is defined as the deviation from an ideal flat surface. It consists of the three measurements of roughness,lay (different from that defined in the wire and cable industry) and waviness and it is is measured with more advanced instruments. As a start I refer you to:

A large variety of surface defects can be observed using an optical microscope. What can be seen is a function of the magnification and the size of the defects as expected.

With magnification up to 50x the usual rolling defects such as seams/laps, drawing defects such as die lines, drawn in slivers (metal fines) and evidence of cold welding or drawn over defects (chevron shaped defect with "V" shape defect parralel to the drawing axis) will also be detected. Excessive grain boundary attack due to excess descaling/oxidation will also be detected - a "pavestone effect"

A variety of mechanical damage due to slipping on capstans, blocks, sheaves etc will also be revealed